Single material optical fiber structures including thin film supporting members

ABSTRACT

Optical fibers, for propagating optical radiation in guided modes, are fabricated in an integral structure. Advantageously, the fiber structure is made of a single filamentary material, such as fused silica, with a relatively large cross section at the central portion of the fiber and with a relatively thin film portion at the extremities of the fiber. The thin film portion has a thickness larger than the wavelength of the optical radiation to be propagated and serves as a supporting member for the central portion of the fiber. Such optical fiber structures are capable of propagating either single mode or multimode guided optical waves. In addition, the exposed surface of the central portion (which is not contacted by the thin film supporting member portion) can be contacted with an optically nonlinear material, in order to provide suitable interactions with the propagating signal wave energy and thereby to produce electrooptic effects such as amplification, modulation, or laser action.

Umted States Patent 1 [111 E 28,664

Miller 5] Reissued Dec. 23, 1975 1 SINGLE MATERIAL OPTICAL FIBER3.712.705 1/1973 Marcatili 350/96 wo STRUCTURES INCLUDING THIN FILMSUPPORTING MEMBERS FOREIGN PATENTS OR APPLICATIONS 2.005570 8/197!Germany 350/96 wo [75] Inventor: Stewart Edward Miller, Locust, NJ.

[73] Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, NJ.

[22] Filed: Feb. 3, 1975 [21] Appl. No.: 546,293

Related US. Patent Documents Reissue of: I [64] Patent No.: 3,813,141

Issued: May 28, 1974 Appl. No.: 308,833

Filed: Nov. 22, 1972 [52] US. Cl. 350/96 WG; 65/DIG. 7; 350/96 R [Sl]Int. Cl. G02B 5/14 [58] Field of Search 350/96 W0, 96 R [56] ReferencesCited UNITED STATES PATENTS 3,318,65l 5/[967 Karbowiak 350/96 WG3,350,654 l0/l967 Snitzer 329/l44 3.391.969 7/l968 Ogle 350/96 B3,434,774 3/l969 Miller 350/96 WG 3,535,017 l0/l970 Miller 350/96 WG3,537,020 10/1970 Anderson 330/45 3,653,739 4/1972 Strack 350/96 BPrimary Examiner-John K. Corbin Attorney, Agent, or Firm-Thomas C.OKonski [5 7] ABSTRACT Optical fibers, for propagating optical radiationin guided modes, are fabricated in an integral structure.Advantageously, the fiber structure is made of a single filamentarymaterial, such as fused silica, with a relatively large cross section atthe central portion of the fiber and with a relatively thin film portionat the extremities of the fiber. The thin film portion has a thicknesslarger than the wavelength of the optical radiation to be propagated andserves as a supporting member for the central portion of the fiber. Suchoptical fiber structures are capable of propagating either single modeor multimode guided optical waves. In addition, the exposed surface ofthe central portion (which is not contacted by the thin film supportingmember portion) can be contacted with an optically nonlinear material,in order to provide suitable interactions with the propagating signalwave energy and thereby to produce electrooptic effects such asamplification, modulation, or laser action.

SINGLE MATERIAL OPTICAL FIBER STRUCTURES INCLUDING THIN FILM SUPPORTINGMEMBERS Matter enclosed in heavy brackets I: 1 appears in the originalpatent but forms no part of this reissue specification; matter printedin italics indicates the additions made by reissue.

FIELD OF THE INVENTION This invention relates to the field of opticalcommunications systems and, more particularly, to optical fiberstructures for the propagation of electromagnetic wave enenrgy.

BACKGROUND OF THE INVENTION ical support uniformly along the wholelength of the optical fibers. In particular, at the juncture of thesupporting member with the optical fiber, an optical disturbance orperturbation is introduced in the modes of wave energy being transmittedthrough the fiber. This perturbation causes various problems in thepropagation of the electromagnetic signal wave energy, such as theunwarranted conversion of signal energy from one mode to another modewith consequent distortion problems. Another problem arises from theneed for cladding" material surrounding the central core of opticalfibers, in order to keep foreign materials (such as dust) fromcontacting the central core and thereby causing further undesiredperturbations of the optical modes propagating through the core. Suchcladding must ordinarily be made of a material having a lower opticalrefractive index than that of the core. For such desirable corematerials as fused silica, it is difficult to find such suitablecladding which has a lower refractive index than the core and, at thesame time, presents sufficiently low optical absorption loss to make itcommercially attractive. Moreover, ordinarily this cladding materialobstructs any coating of the optical fiber core with various opticalmaterials which could serve to provide interaction with the signal waveenergy propagating through the optical fiber, it would, therefore, bedesirable to have available ana optical transmission fiber which issupported in such a way that the cladding of the fiber does notintroduce the losses and obstructions of the prior art.

SUMMARY OF THE INVENTION In accordance with this invention, an opticalfiber, together with transparent supporting members, is made in aunitary integral structure, advantageously formed of a single material.The central portion of the fiber is relatively thick, and is joined attwo or more edges by thin film supporting members of the same materialas that of the central portion. Such a structure can be designed tooperate in either single mode or multimode propagation of the opticalwaves through the fiber. In addition, the exposed periphery ofrelatively thick central portion can be contacted in whole, or at anydesired part, by various linear or nonlinear optical materials, in orderto afford linear or nonlinear interaction with the optical wave energypropagating through the fiber. In this way. further mode control can beprovided, or various devices such as lasers, amplifiers and modulatorscan be integrated into the optical fiber structure.

In a particular embodiment of this invention, a fused silica opticalfiber is fabricated with a rectangular cross section in the centralregion together with a pair of thin film fused silica portionscontiguous with two opposite surfaces of the rectangular cross section.The thickness of the thin film portions of the optical fiber, whichfurnish mechanical support for the central portion, is made relativelylarge compared with the propagating optical wavelength, in order toprovide sufficient mechanical strength for supporting the centralportion of the optical waveguide fiber. On the other hand, thedimensions of the rectangular cross section of the central portion ofthe fiber are made larger than the thickness of thin film supportingmembers, in order that the optical signal wave energy propagatingthrough the overall optical fiber structure is confined to the centralportion thereof by reason of waveguiding properties of the structure.The exposed extreme edges of the thin film supporting members areadvantageously fused to a fused silica glass cylinder which, in turn, iscoated with optically absorbing material. In this way, unwanted "slabguide" modes, which are not exponentially decreasing in intensity in thethin films (going away from the central portion) and which thereby leak"through the supporting members to these edges, are absorbed by thecoating.

Fiber structures of this invention can be fabricated from an originalfused silica fiber of geometrically similar but much larger crosssection than the final desired structure. The original fiber is cleaned,heated and drawn (stretched) in the longitudinal direction, in order toreduce the dimensions of the original cross section to the desiredrelatively small final cross sec tion. The exposed portion of theperiphery of the central region of the final optical structure can be,if desired, then contacted at various locations, uniformly or(spatially) periodic or nonperiodic, with various optically linear ornonlinear materials. Thereby, these materials can provide suitablelinear or nonlinear interaction phenomena along the fiber between theoptical wave energy propagating through the central portion of the fiberand the linear or nonlinear material, In this way, the usefulelectromagnetic signal wave energy propagating through the optical fiberis confined to the central rectangular portion of the fiber; and, at thesame time, access for optically linear or nonlinear materials, tointeract with the propagating signal wave energy, is afforded in theoptical fiber structure of this invention. 1

BRIEF DESCRIPTION OF DRAWING This invention together with its features,advantages and objects can be better understood from the followingdetailed description when read in conjunction with the drawing, inwhich:

FIG. 1 is a longitudinal diagram, partly in cross section, of an opticalfiber structure in accordance with a specific embodiment of theinvention.

FIG. 2 is a cross-sectional view of the optical fiber structure shown inFIG. 1;

FIG. 3 is a cross-sectional view of the optical fiber tructure shown inFIG. 2 in an initial stage of its manutcture; and

FIG. 4 is a cross-sectional view of an optical fiber :ructure inaccordance with another specific emboditent of the invention; and

FIG. 5 is a cross-sectional view of an optical fiber .ructure having acircular central portion, in accorance with yet another specificembodiment of this ivention.

DETAILED DESCRIPTION An optical fiber filament structure (FIG. 1)inludes a transparent central portion 11, and a pair of ipportingtransparent thin film portions 12.1 and 12.2 FIG. 2). The centralportion 11 together with the upporting films 12.1 and 12.2 are locatedin a cavity rovided by a peripheral hollow cylindrical portion 13. noptically lossy jacket 14 advantageously encases the ylindrical portion13. Advantageously, the central ortion 11 and the thin films 12.1 and12.2 are all made fthe same optically transmitting material. Theperiphral portions 13 typically is likewise made of the same laterial asthe central portions of the thin film supportlg members 12.1 and 12.2.An optical source 21 and n optical utilization means 22 are located atopposite mgitudinal ends of the optical fiber structure 10 (FIG.

If desired for nonlinear optical interaction with the ptical wavespropagating through the fiber structure 0, the space cavity region 15between the peripheral ortion 13 and the central portion 11 may befilled with n optically nonlinear material, typically a liquid. In thelternative, the entire exposed surface of the central ortion 11, orvarious portions of said surface, may be sated with optically nonlinearmaterial as desired. In IIS way, the optical wave energy, propagatedfrom the )urce 21 to the utilization means 22 through the optial fiberstructure 10, will advantageously interact with IIS nonlinear material.In addition or as an alternative, 1e entire surface, or various portionsthereof, of the antral portion 11 may be coated with a linear opticallaterial, in order to furnish further waveguiding of the ptical waveenergy propagating through the central ortion II.

The thickness of the supporting thin films 12.1 and 2.2, indicated by bin FIG. 2, is fabricated advantaeously to be larger than the propagatingoptical wave- :ngth furnished by the source 21, in order to providelechanical support for the central portion 11. In addion, the width ofthe supporting thin films 12.1 and 2.2 in the X direction is at least anorder of magnitude trger than the wavelength, in order to providesuffiient space for the exponential decrease of the ampli- 1de of theoptical modes in direction in the :X direcon going away from the centralportion 11. Moreover, is important that the thickness B of the centralporon 11 should be larger than the thickness b of the thin lm portions,in order to provide the desired optical 'aveguiding. In this way theuseful modes propagating trough the optical structure 10 will beexponentially ecreasing with distance in the X direction in theJpporting member 12.1, and also exponentially dereasing in the +Xdirection in member 12.2 (i.e., in ie directions going away from thecentral portion 11 he other optical modes, which are exponentiallyinreasing or periodic in these respective directions in ie supportingmembers 12.1 and 12.2 (slab guide modes), are not useful in thisinvention; and these slab guide" modes are quickly absorbed by theoptically absorbing material of the outer jacket 14 upon theirpropagation through the optical fiber 10 in the 2 direction.

In order to fabricate the optical fiber structure 10 shown in FIGS. 1and 2, it is convenient to start with optically polished fiber opticsegments 31.1, 31.2, 32 and 33 in the fiber structure 30, as shown inFIG. 3. Typically, all of these segments are made of the same opticallytransmitting material such asa fused silica. In order to have cleanoptical surfaces, the exposed surfaces of these segments are cleanedsuccessively with solutions of trichlorethylene, acetone, nitric acid(l:l diluted with deionized water) and deionized water. Alternatively,the known hot fire flame cleaning technique may be used to clean thesurface. Advantageously, the overall cross section of the segments inthe structure 30 as initially arranged (FIG. 3) constitutes ageometrically similar, but greatly enlarged, cross section of thefinally desired cross section shown in FIG. 2. These segments 31.1,31.2, 32 and 33 are heated to a temperature sufficient to fuse themtogether and to enable them to be drawn (stretched) in the longitudinal2 direction, in order to reduce their cross sections to the finallydesired value for the optical fiber 10. Thus, it is to be understoodthat FIG. 3 is not drawn to scale with respect to FIG. 2, but thatordinarily the structure 30 is many times larger in cross section thanthe structure 10.

It should be understood that the relative values of A, B, and bdetermine the number of modes which can be guided by the optical fiberstructure 10 (substantially independent of optical wavelength). The modesupporting efficiency e of guidance of the optical fiber structure 10 isdefined as the ratio of this number of possible guided modes to thenumber of possible modes which can be guided by a similar optical fiberstructure but with b= 0.

EXAMPLE I (Multimode Fiber): An optical fiber structure, with a modeefficiency e of about l0 percent or more, can be afforded by thefollowing choices of parameters. The material for the optical fibermembers 11, 12.1, 12.2 and 13 is selected to be of used silica(refractive [inded 1 index, n L46), for propagating optical radiationfrom the source 21 (wavelength of about one micron) to the opticaldetector 22. The space 15 is filled with air or vacuum (n 1.00) and thethickness b is selected to be about l.4 micron [or less 1 Forpropagating a suitable number of optical modes (multimodes), thedimensions of A and B are typically selected to be at least severaltimes larger than b, but are otherwise arbitrary. For example, A and Bcan be selected in the range of about 5 to 25 micron. It should beremarked, however, that there is an advantage of using a square crosssection (A B), namely, that splicing is made easier in that theunavoidable minor alignment errors in any splicing procedures are not socrucial as for other cross sections (in which A and B are not equal).

EXAMPLE 2 (Multimode Fiber):

For an optical fiber structure 10 with an optical dispersion of no morethan 10 manoseconds per kilometer, the following design can be used.Again, as in Example l, the optical fibers 11, 12.1 and 12.2 are allmade of fused silica (n L46); the space is filled with air or vacuum (n1.00); and the optical source 21 provides a beam of radiation having awavelength of about I micron. For this case, b is selected to about 5.4microns, in order to achieve the desired multimode operation with thedesired dispersion. This choice of parameters will then also provide anumerical aperture (N.A.) of 0.065 radians in the optical fiberstructure 10, and a tolerable radius of curvature (R) of approximately19 millimeters. By numerical aperture is meant the maximum angle ofobliqueness in the optical propagation vector which will be radiatedfrom the output end of the fiber, or which will be accepted by the fiberat the input end; and by tolerable radius of curvature is meant theminimum radius of curvature for the fiber (going around bends forexample) consistent with losses below one percent per centimeter oflength. Typically, for optical propagation in a suitable number ofmultimodes, both A and B are selected to be at least several timeslarger than b, for example, in the range of about to micron.

In this multimode case (for which b= 5.4 micron) moreover, the crosssection of the central portion alternatively can be circular asindicated in FIG. 5, thereby providing a cross section which is easierand less critical for splicing one longitudinal section of the opticalfiber with the next adjacent section. The diameter of this centralportion 51 can be about 75 micron, and the periphery portion 53 can havean inner diameter of 100 micron and an outer diameter of about I50micron. Supporting films 52.1 and 52.2, as well as an optically lossyjacket 54, serve the same function as the films 12.1 and 12.2 and thelossy jacket 14 in P16. 2. A portion of the exposed surface of thiscentral portion can be coated with an optically linear material 56 inorder to provide further waveguiding of the optical radiationpropagating through the fiber. In addition, or alternatively, a portionof the exposed surface of this central portion can be coated with anoptically nonlinear material for interaction with the optical radiationpropagating through the fiber.

It should be emphasized that the above Examples l and 2 providemultimode optical propagation. For some fiber optical systemapplications, however, single mode propagation may be desired. In singlemode operation, as known in the art, only the fundamental modes (withboth polarizations) are propagated by the optical waveguide, which justcuts off for the next (second) higher order mode. A further desirable,though not necessary, condition for single mode operation is that thethickness b (in the Y direction) is much greater than the wavelength ofthe fundamental wave energy, just as for multimode operation asdescribed in Examples l and 2. The cutoff condition, that is therequirement for single mode, is given by the relation 2 2 2 k,,+k (6,1)5tan(k a/2) =e,.k,,k,, (3) B2: B20 B211.

In Equations (2) (4), A, and B, are given by where A, is the vacuumwavelength of the optical radiation and where e, is the correspondingdielectric constant of the optical fiber portion 11 relative to theregion 15.

Whereas Equations (2) (6) apply only to the rectangular cross section inthe range x M2, Equation (1 is perfectly general for any cross sectionprovided the contour in the region x A/2 is sufficiently slowly varyingso that substantially no optical reflection occurs whithin this region(except at the extremities). In this general case, the B in Equation (1is the transverse wave number of the second order mode.

It should be noted that for single mode operation, asymmetricalconfigurations (in the y direction) for the optical fibers in thisinvention are preferred. As illustrated in FIG. 4, an asymmetricaloptical fiber 40 is illustrated which can advantageously be produced, aspreviously indicated in connection with FIG. 3, except that centralsegment 31.2 is omitted entirely. In the embodiment illustrated in FIG.4, it should be noted that the bottom surface of the optical fiber 40 iscompletely planar and has the advantage of fewer intial segments in thefabrication process, as well as the advantage of simpler calculationsfor predicting the optical modes which can be supported in the fiber. 1nthe optical fiber 40, as finally produced, the central portion 41 has atotal thickness in the y direction denoted by D. The central portion issupported by the relatively thin film members 42.1 and 42.2, both ofthickness h, on either side thereof, respectively.

EXAMPLE 3 (Single Mode Fiber):

Again assuming that the rectangular optical fiber 40 is selected to bemade of fused silica, and that the optical radiation to be propagatedtherethrough has a wavelength of approximately I micron, and selecting awidth A which is equal to the thickness D, it follows from Equation (lthat b A/ 2 approximately, for the case where b is at least severaltimes greater than the optical wavelength. While this latter conditionsimplifies the calculation, it is not essential to the invention. In anillustrative case, b is selected to be about 5 micron, with A and Dselected to be about 7 micron.

EXAMPLE 4 (Single Mode Fiber):

Referring to FIG. 4, a single-mode, single-material fiber 40 can supportpropagating optical energy of wavelength approximately I micron. In anillustrative case, the thicknesses of the thin portions 42.1 and 42.2are both about 7 micron, whereas the thickness D of the central portion41 in the Y direction is about 10 micron. Likewise, the width A in the Xdirection of the central portion 41 is also about 10 micron Finally theinner diameter of the periphery portion 43 is about micron, and theouter diameter thereof is about I00 micron.

It should be understood that although the invention has been describedin terms of detailed embodiments, various modifications can be made bythe worker of ordinary skill in the art without departing from the scopeof the invention [it range] For example, vari ous glass materials, inaddition to fused silica, can be used for the transparent materials inthe portions 11, 12.1, 12.2 of optical fiber structure 10. Theseportions need not all be of the same transparent material, so long asthey can be fused together. Moreover, to furnish optical interactionwiwth the propagated modes,

suitably optically nonlinear material to be placed in :ontact with thecentral portion 11 can be selected from such well-known materials asRhodamine 6-G in water, Rhodamine 6-G in methanol, ethyl alcohol,:hlorobenzene, and carbon disulphide. Alternatively, Jr in addition,optically linear material for cladding the :entral portion 11 can beselected of known optically .inear dielectric materials. Also, the crosssection of the :entral portion of the optical fiber need not berectangular, but other contours can be used, such as cireu larly orsemicireularly cylindrical, for both single and multimode operation.

Finally, an optical cable, containing many similar optical fibers ofthis invention, can be fabricated by incorporating these fibers in asingle peripheral structure having a circular, elliptical or rectangularcavity for containing these fibers (all of which are joined to theperipheral structure at the tips of the tin film supporting members).

THEORY Assuming, in the structure shown in FIG. 2, that both A, B and bare all much larger than the propagating optical wavelength, by at leastan order of mangitude, the mathematical solution of the optical boundaryvalue problem presented by this cross section shows that the modes whichare exponentially decreasing in intensity in the i X direction, goingaway from the central portion, can be supported by the structure.Moreover, in discussing these modes, it is convenient to introduce aquantity V defined as wherein n is the common refractive index of thecentral portion 11 and its members 12.1 and 12, at wavelength A where nis the refractive index of the space 15 contacting the exposed surfacesthereof. In terms of this quantity V, it can be shown that the number ofguided modes is equal to N given approximately by (for large numbersthereof only):

The mode supporting efficiency e (the ratio of the number of modesguided by this structure to the same structure except with b 0) is givenby e=l/1+(2V/ It is further convenient to define quantity 8 as follows:

8 (e/2)(ln /n (10) It should be noted that the electromagnetic boundaryvalue problem presented by the structure shown in FIG. 2 can beapproximated as a one-dimensional optical fiber problem with a centralrectangular slab portion, of refractive index n, contacted at only twoopposite sides by rectangular slabs of refracted index n, and by vacuumon the other two sides in which r' and in which n is the refractiveindex of the material in the central portion 11 in FIG. 2. It can befurther shown that in terms of this equivalent problem:

8 n a /n N.A.=n V28. (1

Also, for this structure, the tolerable radius of curvature, R (for onepercent loss per centimeter), is given by:

And the dispersion, in terms of time delay between lowest and highestorder modes per unit longitudinal length of the optical filter, is givenby T n 8/c where c is the speed light in vacuo. The important operatingparameters given by Equations (8) through (15) are thus simplycalculated in advance, in order to design and obtain the structuresdesired operational characteristics.

What is claimed is:

1. An optical fiber structure for waveguiding optical radiation whichcomprises a filament of a unitary optically transparent structure whosecross section is characterized by a relatively thick [cross sectioncrosssectional area portion of a fiber optical material at a centralportion thereof and by relatively thin cross-sec tional area portions ofthe same material contacting at least two extremities of the centralportion, said thin portions having thicknesses larger than thewavelength of the optical radiation and providing mechanical support forthe central portion, and said thin portions extending in a directionaway from the central portion for distances which are at least an orderof magnitude larger than the wavelength of the optical radiation,whereby the optical radiation can be propagated in at least one modethrough the optical fiber.

2. An optical fiber structure according to claim 1 in which the centralportion has a rectangular cross section.

3. An optical fiber structure according to claim I in which the centralportion has a circular cross section.

4. An optical fiber structure according to claim 1 in which the filamentismade of a single transparent mate rial.

5. An optical fiber structure according to claim 1 in which the exposededges of the filament, located distally from the central portion, areattached to a peripheral cylindrical hollow second filament which istransparent to the optical radiation.

6. The optical fiber structure recited in claim 5 in which thecylindrical portion is encased in an optically lossy jacket.

7. The optical fiber structure recited in claim 5 in which at least aportion of the space between the peripheral cylindrical portion and thecentral portion is occupied by an optically nonlinear material forinteraction with the optical radiation propagating through the opticalfiber.

10 rial in order to provide further waveguiding of the optical radiationpropagating through the fiber.

10. An optical fiber structure according to claim 1 in which the centralportion has a rectangularly shaped cross section, and in which one majorsurface of each of the thin portions together with the one major surfaceof the central portion form a single planar surface.

1. An optical fiber structure for waveguiding optical radiation whichcomprises a filament of a unitary optically transparent structure whosecross section is characterized by a relatively thick (cross section)cross-sectional area portion of a fiber optical material at a centralportion thereof and by relatively thin cross-sectional area portions ofthe same material contacting at least two extremities of the centralportion, said thin portions having thicknesses larger than thewavelength of the optical radiation and providing mechanical support forthe central portion, and said thin portions extending in a directionaway from the central portion for distances which are at least an orderof magnitude larger than the wavelength of the optical radiation,whereby the optical radiation can be propagated in at least one modethrough the optical fiber.
 2. An optical fiber structure according toclaim 1 in which the central portion has a rectangular cross section. 3.An optical fiber structure according to claim 1 in which the centralportion has a circular cross section.
 4. An optical fiber structureaccording to claim 1 in which the filament ismade of a singletransparent material.
 5. An optical fiber structure according to claim 1in which the exposed edges of thE filament, located distally from thecentral portion, are attached to a peripheral cylindrical hollow secondfilament which is transparent to the optical radiation.
 6. The opticalfiber structure recited in claim 5 in which the cylindrical portion isencased in an optically lossy jacket.
 7. The optical fiber structurerecited in claim 5 in which at least a portion of the space between theperipheral cylindrical portion and the central portion is occupied by anoptically nonlinear material for interaction with the optical radiationpropagating through the optical fiber.
 8. The optical fiber structurerecited in claim 1 in which at least a portion of the exposed surface ofthe central portion is coated with an optical nonlinear material forinteraction with the optical radiation propagating through the opticalfiber.
 9. The optical fiber structure recited in claim 1 in which atleast a portion of the exposed surface of the central portion is coatedwith an optically linear material in order to provide furtherwaveguiding of the optical radiation propagating through the fiber. 10.An optical fiber structure according to claim 1 in which the centralportion has a rectangularly shaped cross section, and in which one majorsurface of each of the thin portions together with the one major surfaceof the central portion form a single planar surface.